1. Field of the Invention
This invention is related generally to seismic data acquisition systems and more particularly to systems for use in Common Depth Point profiling. The invention is concerned with a solid-state electronic roll-along switch for sequentially connecting successive subpluralities of seismic detectors to a set of recording channels.
2. Description of the Prior Art
Common depth point (CDP) profiling is standard practice in seismic exploration for hydrocarbons. CDP methods are described in numerous patents and technical papers such as U.S. Pat. Nos. 2,732,906 to Mayne, 3,317,828 to Mendenhall, 3,240,286 to Musgrave, 3,775,738 to Quay, and many others.
To review briefly, a large number of seismic detector groups, on the order of several hundred, are distributed in a spread along a line of survey. The detector groups, located at desired stations, are generally separated by uniform group intervals such as 100 meters. A recording unit such as a truck is provided that houses a data acquisition system that includes a number of signal processing and recording channels, usually between 50 and 100 such channels. An acoustic source, mounted on a suitable vehicle, accompanies the recording vehicle.
The recording unit and the source occupy a first recording station. A subplurality of seismic detector groups, usually equal to the number of recording channels, are coupled to the respective channels by suitable electrical connectors. The source radiates an acoustic signal into the earth and a first recording is made of the reflected seismic signals. The recording and source vehicles advance to the next recording station where a second recording is made. The vehicles continue to advance along the line of survey after each recording until the line of survey is completed. The distance between recording stations is usually a multiple of the detector-station spacing. At each new station, the recording channels are disconnected from the previously-connected subplurality of seismic detectors and are connected to a new subplurality. If the separation between recording stations is for example, equal to one group interval, for each subsequent recording, one group is disconnected behind the recording vehicle and a new detector group is connected ahead of the recording vehicle in the direction of advance.
It is, of course, possible to connect and disconnect the detector groups by means of suitable multiconductor plugs at the recording vehicle itself. The recording vehicle would be obliged to physically proceed down the line of survey, stopping at each recording station. It is more practical however, to locate the recording vehicle at a fixed location at one end of the spread and to advance the recording channels along the detector spread electrically, by use of a roll-along switch. In this arrangement, only the acoustic source is advanced along the spread.
Roll-along switches are available commercially from such suppliers as Geospace Inc., or Input-Output, Inc., both of Houston, Tx. Known roll-along switches are mechanical and are operated either by hand or by means of a stepping motor. In general, such switches consist of a movable contact block having contact pairs equal in number to the number of recording channels. The movable contact block mates with a stationary contact block which includes a plurality of mating contact pairs that are connected to the corresponding detector group. A typical roll-along switch might include forty eight contact pairs for the movable block and 196 contact pairs for the stationary block. After each recording is completed, the movable contact block is incrementally shifted one or more detector groups relative to the stationary contact block.
Because of the large number of contacts involved, a considerable amount of friction must be overcome when shifting the movable contact block relative to the stationary block. Automatic switches require relatively complicated shifting mechanisms. Being mechanical, the switch contacts are subject to wear and corrosion. When used in hostile climates, sand, dirt and moisture cause frequent switch malfunctions and often create serious interchannel crossfeed between contacts. Mechanical roll-along switches are also very difficult to repair in the field.
Constraints on the physical size of the switch, limit the number of seismic detector input channels that can be accomodated by a mechanical switch. Known switches provide for about 196 input channels whereas facilities for use with up to 512 input channels or more and up to 120 recording channels are desired. The sheer bulk of such a large number of contacts precludes use of a mechanical switch having such high contact multiplicity. Commercially available roll-along switches are large, heavy and awkward to handle. One such commercial unit having but 196 input channels weighs over 60 pounds.